Dependence of Co anisotropy constants on temperature, processing, and underlayer

The dependence of the Co anisotropy constants on the measurement temperature, deposition conditions, and underlayer materials was studied using unicrystal $\mathrm{Co(101̄0)/Cr}$ (or NiAl)(112)/Ag(110) films sputter deposited on hydrofluoric acid etched Si(110) substrates. The single, in-plane easy axis orientation in these films allows the direct determination of the anisotropy constants. The anisotropy constants of unicrystal Co films are smaller than those of a bulk Co single crystal, and the temperature dependence of the anisotropy constants is stronger. The $K_1$ value drops by 50% as the temperature is increased from 25 °C to 75 °C, and then becomes negative at 135 °C. This zero-crossing temperature is considerably lower than the 250 °C at which $K_1$ of a bulk Co single crystal decreases to zero. The anisotropy constants also vary with the film preparation substrate temperature. Applying a substrate bias during the Co deposition effectively increases $K_1$ to near bulk material values. Unicrystal Co films grown on NiAl/Ag/HF–Si(110) show smaller $K_1$ as compared to those on Cr underlayers. The addition of a thin Cr intermediate layer on the NiAl underlayer, however, restores $K_1$ to the larger value obtained on Cr/Ag/HF–Si(110).